Water in the atmosphere also acts as a greenhouse gas. The atmosphere contains a lot of water. This water can be in the form of a gas—water vapor—or in the form of a liquid—clouds. Clouds are water vapor that has cooled and condensed back into tiny droplets of liquid water.
Water in the clouds holds in some of the heat from Earth's surface. But the bright white tops of clouds also reflect some of the sunlight back to space. So with clouds, some energy from the Sun never even reaches Earth's surface.
How much the clouds affect the warming or cooling of Earth's surface is one of those tricky questions that several NASA missions are aiming to answer.